Add a mode for checking the diff only for clang_tidy_tool.py

Usage:
	chromium/src$ git show -U5 | tools/clang/scripts/clang_tidy_tool.py --checks=* --diff out/Debug chrome
or
	chromium/src$ git diff -U5 | tools/clang/scripts/clang_tidy_tool.py --checks=* --diff out/Debug chrome


Add `--diff` mode to clang_tidy_tool.py to check only the modified lines of code.
Can be used as a kind of presubmit routine.

Will run much faster (parallelized) after my https://reviews.llvm.org/D57662 is landed in LLVM.

Bug: 936989
Change-Id: I83369d3479f271ed62a757b42fea2e22802f9569
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1514052
Commit-Queue: Zinovy Nis <zynis@yandex-team.ru>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640229}
1 file changed